1. Field of the Invention
The present invention relates to a method of forming satisfactory composite images or dichromatic copy images free from fog.
2. Description of the Prior Art
To comply with diversified modes of processing information in recent years, copying machines have been devcloped for forming composite copy images by forming on an electrostatic latent image bearing surface a first latent image and a second latent image successively or simultaneously, the second latent image being opposite in polarity to the first latent image or having the same polarity as the first image but being different therefrom in potential, developing the first and second latent images thus formed as a composite image to obtain a toner image, and transferring the toner image to the surface of a transfer material.
Published Unexamined Japanese patent application No. SHO 57-8553 discloses a copying machine of the type described above which is adapted to form a composite electrostatic latent image as shown in FIG. 1a. More specifically the photosensitive member is charged to a predetermined surface potential Vs and then exposed to a positive image to form a first electrostatic latent image. The amount of exposure at this time is such that the resulting background area has an intermediate potential VL which is approximately a half of Vs. The area of the intermediate potential VL is then exposed to a negative image to form a second electrostatic latent image represented by a potential VLL.
Sometimes, however, there arises a need to develop the first and second latent images with toners of different colors for the purpose of edition, discrimination or the like. In this case, it is usually necessary to use two developing units and therefore to use a copying machine of correspondingly increased size. This drawback can be overcome by using a single developing unit for developing the first and second latent images with toners of different colors.
In this connection, the present applicant already proposed in U.S. patent application Ser. No. 535,933 filed on Oct. 27, 1983 to develop such composite latent images in two colors with a three-component developer comprising a magnetic carrier, a magnetic toner and a nonmagnetic toner. According to the proposal, the composite electrostatic latent image shown in FIG. 1a is developed by the magnetic brush process using a positively charged black toner as the magnetic toner and a negatively charged red toner as the nonmagnetic toner. The process is practiced with application of a bias voltage set to a level somewhat higher than the intermediate potential VL as shown in FIG. 1b, whereby the red nonmagnetic toner is deposited on the first latent image and the black toner on the second latent image. Moreover the nonmagnetic toner is deposited on the edge portion of the second latent image of the negative due to an edge effect because the nonmagnetic toner is not restrained magnetically unlike the magnetic toner and is therefore prone to deposition. Nevertheless, the transferred image is susceptible to fogging. Further when the developer containing a magnetic toner and a nonmagnetic toner is used as above, the magnetic toner is generally less amenable to development owing to magnetic restraint, with the result that a difference occurs in the image density between the two toners as seen in FIG. 1b.
These drawbacks are experienced also when dichromatic originals are copied in two colors. Thus, it has been difficult to obtain satisfactory copy images.